Waveguide filter



March 25, 1969 D. RENKOWITZ WAVEGUIDE FILTER Filed May 28 F F F F Frequency FILTER BAND PASS Port 2 3 db DIRECTIONAL COUPLER POWER OUT Port 1 Per? 4 M/VENTOR DONALD RENKOWlTZ BY 44;. ,1. F4. AT ORNEX United States Patent U.S. Cl. 333-73 6 Claims ABSTRACT OF THE DISCLOSURE A microwave band reject filter is described wherein a four port directional coupler is provided with a bandpass filter and a quarter-wave stub at one port and short-circuiting end wall at another port. In operation, the band pass characteristic of the filter is in effect inverted to provide band rejection.

This invention relates to a waveguide filter and more particularly to a waveguide band reject filter.

In the transmission of high-frequency or microwave signals, the rejection of undesired signals having frequencies differing from that of a desired signal is generally attained by the use of a multisection, narrow band, band-pass filter. As known in the art, a band-pass filter severely attenuates signals having frequencies residing outside the particular pass band with the attenuation being a function of the difference between the center frequency of the pass band and the signal frequency. However, a certain amount of attenuation is present even at frequencies within the pass band of the filter and this attenuation is found to increase as the width of the pass band is decreased.

An alternative solution to the suppression of undesired signals is to employ a band reject filter which is designed to reject all signals having frequencies within a particular band, while passing all other frequencies. By selecting the reject band of the filter to include the undesired signal and exclude the desired signal, the attenuation of the desired signal is minimized while the required rejection is attained.

Although band rejection is desirable, in practice it 'has been found ditficult to construct a band reject filter for microwave signals. The lumped-circuit equivalent of a band reject filter generally comprises a parallel resonant circuit connected in series with the transmission line. However, due to the distributed nature of the electromagnetic fields in a Waveguide, it has been found difficult to incorporate series elements in a waveguide.

Previous methods of providing a waveguide band reject filter have utilized a tee-section Waveguide in which one wall of the waveguide is broken and a stub is connected thereto. A band-pass filter, the pass band of which is equal to the band to be rejected, is mounted in the stub to in effect pass the undesired signals through the stub. However this method of utilizing a parallel resonant shunt for band rejection has been generally found unsatisfactory due to the lack of symmetry in the waveguide, since only one wall is opened, and to the lack of physical isolation between the input and output. This lack of isolation results in a fringing of the fields at the opening in the waveguide wvall thereby reducing the rejection obtained.

Accordingly, an object of the present invention is to provide means for rejecting undesired microwave signals while passing desired signals essentially without attenuation.

Another object is to provide an improved waveguide band reject filter.

In accordance with the present invention, a directional coupler having first, second, third and fourth coupling ports, i.e. openings at the ends thereof through which a signal may enter or leave the waveguide device, is connected to the output of a waveguide structure by means of its first coupling port. Generally, a directional coupler is a device comprising a primary system and a secondary system having coupling therebetween, wherein the direction of the signal coupled into the secondary system is determined by the direction of the signal in the primary system. In one embodiment utilizing a conventional twohole directional coupler, the primary system consists of the waveguide path between the first and second ports with the secondary system being between the third and fourth ports. And, without considering the effect of any reflected signals from the ports, a signal travelling from the first port to the second port will provide an output at the second and third ports. Signals, travelling from the second port to the first port provide an output at the first and fourth ports. This coupling relation similarly applies to signals travelling between the third and fourth ports.

In addition, the portion of the signal coupled into the secondary system is a particular fraction of the primary signal and is shifted in phase with respect thereto. For a 3 db degree directional coupler, the voltage coupled into the secondary is .707 that of the incident primary voltage shifted in phase by an angle of 90 degrees.

A band-pass filter is connected to the second coupling port of the directional coupler with the pass band thereof being equal to the desired reject band. At the output of the filter, a short-circuited quarter wavelength stub is provided. Therefore, signals travelling from the first to the second port and having a frequency within the pass band of the filter are reflected therefrom due to the shortcircuit appearing as an open circuit at the filter output. However, for signals outside the pass band of the filter, the filter appears as a short circuit and these signals are reflected and phase shifted by degrees.

The third coupling port of the directional coupler is terminated by a short-circuiting end wall which reflects all incident signals and provides a 180 degree phase shift. The fourth and remaining coupling port is the output port and is connected to the particular load.

The above described combination comprises a bandreject waveguide filter with signals incident at the first port providing an output at the fourth port essentially only for input signals having a frequency residing outside the pass band of the filter provided at the second port. This condition is obtained by the cancellation or, alternately, the reinforcement of the signal reflected from the short-circuited third port to the fourth or output port by the signal reflected from the band-pass filter at the second port. Whether an output signal appears at the fourth coupling port depends upon whether the signal reflected from the band-pass filter at the second port is shifted in phase by 180 degrees. Input signals having a frequency within the pass band of the filter are reflected without experiencing a phase shift and when coupled into the secondary system reuslt in essentially no output at the fourth port. The opposite condition is obtained for input signals without the pass band.

This overall method of band rejection is due to the effective inversion of the pass band of the filter at the second port. It will be noted that the inversion needed for an overall band rejection characteristic is provided without the desired signal entering the band-pass filter. Thus, any power dissipated at this frequency is that normally encountered in waveguides and is essentially independent of the dissipation in the band-pass filter.

Further features and advantages of the invention will be readily apparent from the following discussion of specific embodiments when viewed in conjunction with the accompanying drawings in which:

FIG. 1 is a perspective view in section of one embodiment of the invention;

FIG. 2 is a block schematic diagram of the embodiment of FIG. 1;

FIG. 3 is a curve showing the transmitted powerfrequency characteristic of the band-pass filter in the embodiment of FIG. 1;

FIG. 4 is a curve showing the over-all transmitted power-frequency characteristic of the embodiment of FIG. 1; and

FIG. 5 is a perspective view of a second embodiment of the invention.

Referring now to FIG. 1, a waveguide band-reject filter is shown comprising a directional coupler having four coupling ports, a band-pass filter 13 connected to one of the coupling ports and a quarter-wavelength section 16 of waveguide terminated in a short circuit and connected to the output of the filter 13.

The directional coupler is shown having four sequentially numbered coupling ports with the waveguide path from the input port 1 to port 2 herein termed the primary path and the path from port 3 to the output port 4 termed the secondary path. The coupler 10 is a directional device wherein a portion of a first signal propagating along either the primary or secondary path is coupled through coupling slots 11 and 12 to the secondary or primary path. In eflect, the first signal is divided into two components each of which propagates in the same direction as the first signal. The relative amplitudes of the two components are determined by the coupling coefficient of coupler 10. And, for a 3 db coupler the first signal is divided into two components each having an amplitude equal to .707 that of the first signal. In addition, the coupling slots 11 and 12 are spaced an odd number of quarter wavelengths apart to prevent any portion of the signal from travelling in the opposite direction. This coupling between the primary and secondary paths results in the coupled component experiencing a phase shift of 90 degrees relative to the non-coupled component. Further discussion of this type of directional coupler may be found in Electronic and Radio Engineering, 4th ed., by F. E. Terman, at page 156.

As shown in FIG. 1, port 3 is terminated by short circuiting end wall 18 which reflects any incident signals and provides a 180 degree phase shift therefor. In addition, port 2 of the directional coupler is connected to band-pass filter 13.

Band-pass filter 13 is a waveguide cavity resonator having an electrical length of substantially one-half a wavelength at the center frequency of the pass band with coupling irises 14 and 15 located at the input and output respectively. The transmitted power-frequency characteristic of filter 13 is shown in FIG. 3 with its pass band, defined by frequencies and f centered about center frequency f The design considerations for cavity resonators of this type may be found in Fields and Waves in Modern Radio, 2nd ed., by S. Ramo and J. Winnery, at page 241. It will be noted that many types of waveguide band-pass filters may be employed including a multi-section filter. And as known in the art, a bandpass filter passes a signal having a frequency residing within its pass band while rejecting signals having frequencies outside the band.

The output of band-pass filter 13 is connected to a quarter-wavelength section 16 of. waveguide which is terminated by short-circuiting end wall 17. The length of section 16 is made substantially equal to one-quarter of the waveguide wavelength at the center frequency of the pass band of filter 13 so that signals passed by the filter see in effect an open circuit at the output thereof. Thus, signals at port 2 having a frequency within the pass band of filter 13 are reflected from the filter by the terminating open circuit while signals having a frequency outside the pass band, such as frequency 3 in FIG. 3, do not enter the band-pass filter and are reflected with a 4 phase shift of 180 degrees. This is due to the fact that the filter appears as a short circuit to signals having a frequency sufficiently outside the pass band.

The signals reflected by band-pass filter 13 travel along the primary path from port 2 to port 1. However, a portion of this signal is coupled into the secondary and travels toward output port 4. And, for a 3 db coupler the reflected signal is divided into two components of equal magnitudes with the coupled component in the secondary being shifted in phase by 90 degrees.

The band rejection of the over-all device will be more readily understood with the aid of the block diagram of FIG. 2 wherein the signals appearing at the ports and at the output of the band-pass filter are denoted by a and b depending upon their direction of propagation.

The application of input signal a at port 1 results in incident signals at ports 2 and 3. The amplitude of these signals a and a is equal to .707 a due to the 3 db coupling provided by the directional coupler. In addition, the difference in path lengths between port 1 and ports 2 and 3 results in signal a having a phase angle of 90 degrees with respect to a Further, the incident signal (1 at port 3 is reflected by short-circuiting end wall 18 with an additional phase shift of 180 degrees so that [2 equals a The reflected signal b at port 3 in turn provides an output at ports 1 and 4. The output at port 4 is equal to .707 b or .5 a with a phase angle of -90 degrees with the output of port 1 equal to .707 b having a phase angle of 90 degrees or .5 a In summary, the signal reflected at port 3 provides output signals of equal amplitude at ports 1 and 4 with the signal at port 4 having a relative phase angle of -90 degrees.

The primary component a which is incident at port 2 encounters band-pass filter 13. If the frequency of the primary component resides outside the pass band of the filter, such as frequency i of FIG. 3, the filter appears as a short-circuit which reflects the incident signals and provides a l degree phase shift. However, primary components having a frequency within the pass band of the filter are not reflected thereby. These components are transmitted by the filter which in turn is connected to a quarter wavelength section of waveguide 16 terminated by a short-circuiting end wall 17. The short-circuited section of waveguide reflects the signals passed by the filter without a change in the phase angle due to the electrical lengths of section 16 and filter 13.

In summary, the reflected component b at port 2 has an amplitude equal to .707 12 and is shifted in phase by 180 degrees if its frequency resides outside the filter pass band. The reflected component [2 provides an output at port 1 equal to .707 b and an output at port 4 equal to .707 b having a phase angle of degrees. These outputs then combine with the reflections from port 3.

At port 4, the combined output signal b; is the sum of .707 11 having a phase angle of 90 degrees and .707 h which as noted previously equals --.5 al having a phase angle of -90 degrees. For primary components having a frequency without the pass band, .707 15 at a phase angle of 90 degrees equals .5 a at a phase angle of --90 degrees. Thus, the amplitude of the combined output signal b is equal to the input signal a for frequencies without the band. Thus, essentially all energy at these frequencies is passed from port 1 to port 4 without loss.

Further, primary components having a frequency within the pass band are reflected by the effective open circuit at the termination of the filter so that the reflected component b equals .707 a When component b is coupled into the secondary path, it appears as .5 a having a phase angle of 90 degrees. Thus the combined output signal b; is equal to zero for frequencies within the band and essentially loss-less band rejection is attained.

The response of the band-reject filter is shown by the overall transmitted power-frequency characteristic of FIG. 4. It will be noted that this curve is the inverse of that shown in FIG. 3 and that the width of the overall band rejection characteristic is thus determined by the bandpass characteristic of filter 13. In one embodiment employing a band-pass filter having a center frequency f of 6,000 mc./s. and a bandwidth of 30 mc./s., the insertion loss due to the overall band rejection was found to be 30 db corresponding to the null depth at frequency f in FIG. 4. However, the desired signal, f;, of 6,700 mc./ s. was found to experience a loss of only 0.10 db. This low loss is due to the fact that the desired signal f does not enter the band-pass filter and thus the loss is not dependent on the dissipation in the band-pass filter.

A second embodiment is shown in FIG. 4, wherein a hybrid or magic T waveguide section is employed as the directional coupler 10 of FIG. 1. As known in the art, a hybrid T is a section of waveguide having two branches added at the same point. A signal applied to either one of the branches provides a signal at the two outputs of the rectangular waveguide and no signal at the other branch.

The hybrid T section 10' is a four port device, sequentially numbered in FIG. 5, with the Waveguide paths between ports 1 and 2 and ports 3 and 4 being termed herein the primary path and secondary path respectively. As shown, port 1 is the input port and a signal applied thereto is divided into two components of equal amplitude at the junction, one of which travels along the primary path to port 2 with the other being coupled to the secondary path and travelling to port 3.

Port 2 is spaced essentially one-half a wavelength from the branch of port 4 and is connected to band-pass filter 13 which in turn is connected to a quarter wavelength section 16 terminated by short-circuiting end wall 17'. The signals travelling along the primary path are reflected at port 2 as previously explained in connection with the embodiment of FIG. 1. In addition, port 3 is spaced essentially three-fourths of a wavelength from the branch of port 4 and is terminated by short-circuiting end wall 18'.

The component of the input signal coupled to the secondary path is reflected by the short-circuiting end wall 18' at port 3 and is 180 degrees out of phase with the primary component having a frequency without the pass band of filter 13. This is due to the difference in path lengths therebetween and results in an output signal at port 4. However, primary components having a frequency within the pass band of filter 13 are not shifted in phase with the reflected secondary component at the port 4 branch. It will be noted that when both reflected signals are in phase at the branch of port 4, no output signal is obtained due to the electric vectors turning a corner at the junction which results in the in phase components cancelling each other. Further description of the hybrid T may be found in the previously cited reference of F. E. Terman at page 156.

While the above description has referred to specific embodiments of the invention, it will be readily understood that many other modifications may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. A waveguide filter for rejecting microwave signals having a frequency within a selected band which comprises (a) a primary conducting path having an input port and an output port,

(b) a secondary conducting path having a first port and an output port,

(c) means for coupling a component of a signal travelling in one of said conducting paths to the other of said conducting paths such that an input signal applied to said primary conducting path is divided into two components of equal amplitude, one of which is coupled to said secondary conducting path,

(d) terminating means connected to the first port of said secondary conducting path for reflecting said coupled component of said input signal and providing a 180 degree phase shift thereto,

(e) a filter connected to the output port of said primary conducting path for passing primary components therein having a freuqency within said selected band and rejecting components of other frequencies, said rejected components of other frequencies experiencing a degree phase shift, and

(f) means connected to said filter for reflecting the primary components passed thereby with substantially no phase shift, said reflected and rejected primary components being coupled to said secondary conducting path whereby the sum of said coupled primary component and said second component provides a substantially zero output signal at the output port thereof for frequencies within said selected band while providing an output signal at frequencies without said band.

2. A waveguide filter for rejecting microwave signals having a frequency within a selected band which comprises (a) a directional coupler having first, second, third and fourth coupling ports, said first and fourth coupling ports comprising the input and output respectively,

(b) means for terminating said third coupling port in a shout-circuit whereby incident signals are reflected thereby and shifted in phase by 180 degrees,

(c) filter means connected to said second coupling port for rejecting signals without said selected band and providing a 180 degree phase shift thereto while passing signals within said selected band, and

(d) means for terminating said filter means in a substantially open circuit to reflect signals passed by said filter means without phase shift, the application of an input signal to the first port of said directional coupler resulting in an output signal at the fourth port thereof essentially only for input signals without said selected band.

3. A waveguide filter for rejecting microwave signals having a. frequency within a selected band which comprises (a) a 3 db directional coupler having first, second, third and fourth coupling ports, said first and fourth coupling ports comprising the input and output respectively,

(b) a short-circuiting termination connected to said third coupling port whereby incident signals are reflected thereby and shifted in phase by 180 degrees,

(c) filter means connected to said second coupling port for rejecting signals without said selected band and providing a 180 degree phase shift thereto while passing signals within said selected band, and

(d) means for terminating said filter means in a substantially open circuit to reflect signals passed by said filter means without phase shift, the application of an input signal to the first port of said directional coupler resulting in an output signal at the fourth port thereof essentially only for input signals without said selected band.

4. A waveguide filter for rejecting microwave signals having a frequency within a selected band which comprises (a) a 3 db directional coupler halving first, second, third and fourth coupling ports, said first and fourth coupling ports comprising the input and output respectively,

(b) a short-circuiting termination connected to said third coupling port whereby incident signals are reflected thereby and shifted in phase by 180 degrees,

(c) a band-pass [filter having an electrical length of one-half wavelength at the center frequency of the pass band thereof, said filter being connected to said second coupling port for rejecting signals without said selected band and providing a 180 degree phase shift thereto while passing signals within said selected band, and

(d) means for terminating said filter in a substantially open circuit to reflect signals passed by said filter without phase shift, the application of an input signal to the first port of said directional coupler resulting in an output signal at the fourth port thereof essentially only for input signals without said selected band.

5. A Waveguide filter for rejecting microwave signals having a frequency within a selected band which comprises (a) a two-hole directional coupler having first, second, third and fourth coupling ports wherein said holes are spaced an odd number of quarter wavelengths apart, said first and fourth coupling ports comprising the input and output respectively,

(b) a short-cirouiting termination connected to said third coupling port whereby incident signals are reflected thereby and shifted in phase by 180 degrees,

(c) a band-pass filter having an electrical length of one 'half wavelength at the center frequency of the pass band thereof, said filter being connected to said second coupling port for rejecting signals without said selected band and providing a 180 degree phase shift thereto While passing signals within said selected band, and

((1) means for terminating said filter in a substantially open circuit to reflect signals passed by said filter without phase shift, the application of an input signal to the first port of said directional coupler resulting in an output signal at the fourth port thereof essentially only for input signals without said selected band.

6. A waveguide filter for rejecting microwave signals having a frequency within a selected band which comprlses (a) a hybrid T directional coupler having first, second, third and fourth coupling ports with said first and fourth coupling ports comprising the input and output respectively, said second and third coupling ports being unequally spaced from said fourth coupling port by a quarter wavelength,

(b) a short-circuiting termination connected to said third coupling port whereby incident signals are reflected thereby and shifted in phase by 180 degrees,

(0) a band-pass filter having an electrical length of on'e half wavelength at the center frequency of the pass band thereof, said filter being connected to said second coupling port for rejecting signals with out said selected band and providing a 180 degree phase shift thereto while passing signals within said selected band, and

(d) means for terminating said filter in a substantially open circuit to reflect signals passed by said filter without phase shift, the application of an input signal to the first port of said directional coupler resulting in an output signal at the fourth port thereof essentially only for input signals without said selected band.

References Cited UNITED STATES PATENTS 3,034,076 5/ 1962 Tomiyasu 3339 3,142,028 7/1964 Wanselow 333-10 3,234,555 2/1966 Petrilla et a1.

2,6795 5/ 1954 Drazy.

3,056,096 9/1962 Vane 333-10 3,210,693 10/1965 De Vos et a1 3339 2,916,712 12/ 1959 Artuso.

HERMAN K. SAALBACH, Primary Examiner.

C. BARAFF, Assistant Examiner.

US. Cl. X.R. 333-9, 10 

